Hi, pilot experience and for your consideration.
Dumb luck. There I was, standing in McDonalds saw CNN, took a seat and watched.
CNN has some facts already wrong. I counted 10-16 floors above crash area, which was a three floor zone as best I could tell from the monitor. The "20th floor of 50 story building" is incorrect, as crash was 10-16 floors low. Hard to due to the building being a beveled, ziggurat type top that allows for a few more floors of garden apartments . . . Also, not able to see where building meets ground from the camera angles. Report was he took off from Teterboro.
Looking down the East River from the TV view, weather not a factor. Ceiling looked to be at least 1000 feet. This is important, as pilot was (reported) not on an instrument flight plan. You need 1000/3 for VFR. The East River has a VFR corridor (I have flown it, late 90's). Wish I had the Terminal Area Chart to show you where I am talking about.
Typically, you have to stay below 800', or at 500' on that corridor to avoid conflicting with La Guardia traffic. To enter the corridor, I recall the need for a squawk (IFF code) from New York Approach, details fuzzy in my memory, then switch to the VFR advisory freq to deconflict "see and avoid" with othe VFR traffic in the corridor.
An eye witness who was (reported) a private pilot (CNN called him an amateur pilot in usual disregard for facts) had just got on the phone and described what he saw as he went over a bridge. (Guessing bridge that crosses the East River at about 52d street. Name escapes me: Tri Borough? ) He said the plane was maneuvering eratically just before impact. From where he sat, plane went behind a/the (didn't understand the words) building and then the smoke/explosion blossom came out.
Marvell Scott, (name familiar, once a baseball or basketball player?) was interviewed a few minutes later. He is now a WABC sports anchor in New York. He saw the plane briefly from the building he was in, which is across the street from the crashed into building. Hyperbole aside, his observations included nose high pitch, apparent avoidance of his building, and a loud crash, which Marvell scurried across to go and see. (Having digested all this, he went to find a news crew, as best I can fathom.) His description was lucid, compared to some witness reports.
CNN claims tower was not in touch with plane. No matter, they would not have been. IIRC, 72d street is well outside (north of) the five mile circle from La Guardia, so he'd have been on the VFR deconfliction frequency. Forget the VHF numbers. (110. something, IIRC, but that might have been the Hudson River VFR freq) "See and avoid" is pilot resonsibility in most VFR corridors, and you usually call on the freq when you enter the corridor to let pilots slower, faster, or coming the other way know that they need to look for you. No idea what rules have changed in the Manhattan area since 9-11.
I see five scenarios, some of which have been proposed by posters above.
1. Heart Attack, or other medical malfunction, erratic control, impact. Likely. Young guys (juiced perhaps?) not immune to heart attack.
2. Engine failure, frantic "aw crap" reaponse as he loses lift/energy and fails to find a place to land. Possible attempts to get to Central Park to land, crash building is West of the River, and the corridor, as is Central Park. However, that scenario would argue for a glancing blow on a building as he tries to glide down 72 street in an attempt to find the Park. (Low odds)
With the river right there to ditch into, not betting the odds on above scenario. Not everyone would do as I would have done and ditched in the river under an engine failure in that situation, since not all persons have benefit of dunker training, etc. )
Also likely, still puzzled as pilot's decisions.
3. Control malfunction. Jammed control or snapped control cable for elevator of aileron. Low odds, though the eratic maneuvering may point to that.
4. The lover/revenge/suicide deal. Funny that others came up with that, Hollywood has corrupted us all!
5. Deliberate attack for reasons that vary: very low odds.
6. The Neo Cons, of course!

They were delivering 4' rods with 3" centers . . . by plane . . . yeah, that's the ticket!
NORAD of course has launched fighters. This incident allows them to see if their procedures and response times are within the zone desired.
Note: NORAD were not able to order a pre emptive launch, only a reaction.
All I have to offer as of this moment.
ETA: That VFR corridor goes past Yankee Stadium. Weird thought about Lidle: one last look at the stadium before he ends it all? I sincerely hope not. Ouch.
DR